An ice dam forms when snow on the upper roof melts, runs down, and refreezes at the cold eave. The melt is the clue. Snow does not melt evenly on a roof that is the same temperature as the outdoor air. It melts where heat from the house is reaching the deck. The neighbor with the same storm and a clean eave usually has a colder attic, not better luck.
The cycle, in order
Warm air leaks through recessed lights, bath fans, the chimney chase, the attic hatch, and the top plates of interior walls. It warms the sheathing. Snow on that sheathing turns to water, which runs until it hits the overhang. The overhang is outside the heated box, so it stays cold, and the water freezes. The next cycle adds to the ridge. Water behind the ridge can back up under shingles and into the house, which is why the leak shows up on a freezing week rather than during the thaw people expect.
Houses with a shallow pitch hold that water longer. A steep roof sheds it. Deep snow also insulates the deck and can feed the cycle. None of this requires a dramatic blizzard. A modest snow followed by a sunny, below-freezing day is enough.
What actually fixes it
Stop the heat first. Air-sealing the ceiling plane is unglamorous and more effective than another product on the eave. Then look at insulation coverage at the perimeter, where batts are often thin exactly above the wall. Then look at ventilation as a pair: soffit intake and ridge exhaust, with a clear channel between insulation and deck. A ridge vent with stuffed soffits does almost nothing. The longer version of that system is how insulation and ventilation work together.
When the roof is replaced, ice-and-water membrane carried well up from the eave, and through valleys, does not stop the dam. It keeps the backed-up water out of the deck when a dam forms. That is worth doing in snow country. It is not a substitute for the attic work.
Heat cable is a last tool
Heat cable can keep a drainage channel open on a problem edge, such as a valley over an unheated porch that you cannot rebuild this year. It uses electricity all winter, it fails, and it does not address the heat loss. Use it where the geometry leaves no other path, not as the default response to a ridge of ice.
Gutters do not cause dams, but a gutter packed with ice makes the mess heavier and can pull the gutter off the fascia. Clean them before the first freeze. Gutter guards reduce leaf load. They do not cancel an attic problem.
Questions readers ask
Will a new roof stop ice dams?
Not by itself. A new roof with better membrane protects the deck. The dams shrink when the attic stays cold. Do both if you are reroofing in a snow climate. Do the attic first if the roof surface is still sound.
Is raking the snow useful?
A roof rake used from the ground can reduce the snow available to melt. It is a seasonal habit, not a repair, and it is easy to scratch shingles if you rake aggressively. Do not climb up to do it.
When should the attic work happen?
Before the freezing season, while the attic is safe to be in and the eaves are not iced. Midwinter is a bad time to discover that the hatch is the main leak of warm air.
Take Care Nest publishes general information for homeowners. It is not an inspection, a bid, or a substitute for a licensed contractor. Where the structure may be unsafe, get a qualified professional on site before anyone walks the roof.